The objective of the study is to answer the question "Does a policy of CEA-prompted second-look surgery following 'curative' resection of colo-rectal cancer produce a decrease in morbidity and mortality due to tumor recurrence, despite sequelae of second-look surgery?" The multicenter trial set up in order to produce a definitive answer to this question will continue to recruit clinicians interested in this subject. Regular monthly CEA assays will be carried out at the Charing Cross Hospital on all patients entered into the study. The Trials Centre staff will continue to carefully monitor both clinical data and CEA assay results. Should a "significant" CEA rise occur (as defined in the Protocol), the patient will be randomized by the Centre's computer but the clinician will only be informed if the allocation is to the "CEA Aggressive" arm. On receiving this information, the clinician in charge will perform an extensive clinical work-up on the patient and, unless evidence of distant incurable spread or a nonmalignant cause for the rise is found, the patient, after obtaining informed consent, will be taken to second-look laparotomy. CEA results will not be available to surgeons unless a patient is randomized to the "CEA Aggressive" arm, thus preventing any bias in clinical follow-ups. The study will involve close liaison between surgeons, medical oncologists, biochemists, and data managers. The trial will produce the following: (1)\a definitive answer to the above question; (2)\an accurate picture of "lead time" produced by CEA compared to clinical data (available from analysis of the "Conventional" arm); (3)\further data relating CEA levels to tumor histology and topography; and (4)\a large database on the natural history of bowel cancer. At the end of May 1984, 217 patients had been registered into the study and 13 randomized (5 "CEA Aggressive" arm, 8 "Conventional" arm). (2)